Conditions InDepth: Leukemia

Cancer is a disease in which cells grow in an abnormal way. Normally, the cells divide in a controlled manner to replace old or damaged cells. With leukemia, blood cells begin to develop abnormally and rapidly divide creating more blood cells than needed. These cells develop in the bone marrow, but will eventually travel throughout the body in the bloodstream. The abnormal blood cells do not function as normal blood cells. They also crowd out normal cells, which can lead to problems with the immune system, oxygen delivery, and blood clotting.

Normal Blood Cells and the Development of Leukemia

All blood cells start as stem cells that are formed in bone marrow. Stem cells can mature into a variety of different blood cell types that have specific functions in the body. These include:

Red blood cells—Carry oxygen from the lungs to the organs and cells of the body.

New, healthy cells are developed in the bone marrow to replace old or damaged cells. This ensures there is a consistent number of blood cells in the body. With leukemia, there is an excessive development of new, abnormal blood cells. The abnormal cells crowd the bone marrow, making it difficult for new, healthy cells to develop. The abnormal cells and the lower levels of healthy cells lead to a weakened immune system, problems problems with oxygen delivery, or problems controlling bleeding depending on the type of blood cells that are affected. Cancerous blood cells also circulate in the blood and lymph systems, affecting organs like the spleen, liver, brain, and lymph nodes.

The instruction for cell growth and cell death exists in the DNA of each cell. Leukemia is the result of damage to DNA. The damage may be the result of genetics, environmental factors like radiation exposure, changes related to age, or a combination of these factors.

Types of Leukemia

Leukemia is distinguished by how fast the leukemia develops and the type of blood cell that is affected. Leukemia may be distinguished as:

Acute—Abnormal cells are immature and the effect on the body is quickly apparent. Acute leukemias progress rapidly.

Chronic—Abnormal cells are mature and look similar to normal cells, but do not function as well. Chronic leukemia may go unnoticed for years until symptoms appear or it is found incidentally during a routine blood test.

Leukemia is also distinguished by the type of bone marrow cells that is starts in:

Revision Information

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.

Overview of leukemia. Merck Manual Professional Version website. Available at: http://www.merckmanuals.com/professional/hematology-and-oncology/leukemias/overview-of-leukemia. Updated October 2014. Accessed February 1, 2016.